Even if DayZ were to become the financial flagship of BIS, I can't see them developing it as the flagship. That is, I don't see them making their next generation engines based on the needs of a zombie survival game (though, perhaps they'll add some features for it, like melee weapons). As a military sim engine, it is very flexible; able to be a civilian helicopter simulator, for example. An engine built for a zombie game likely would make it inflexible. You can make a zombie mod out of a military-based engine, but it'd be very time-consuming to make a milsim or flight sim out of a zombie engine.
I do hope that DayZ doesn't change their focus away from Arma. The good majority of people that I know who bought Arma CO for DayZ, said that they got bored of DayZ relatively quickly and thought they'd try Arma - only to discover that they love it. Maybe it isn't representative of everyone who bought Arma CO for DayZ, but I still think it would be a mistake to neglect Arma. Arma can be so many things, while a zombie game cannot easily.